Ordinarily, in semiconductor device and integrated circuit fabrication processes, the shapes and dimensions of various patterns formed on wafers need to be inspected and evaluated with high precision. Accordingly, not only fabrication devices, but also inspection and evaluation devices are demanded high precision to accommodate the reduced sizes of such semiconductor devices and integrated circuits. As such, for the inspection of the shapes and dimensions of various patterns formed on wafers, scanning electron microscopes (also referred to herein as SEM) with a metrology function are used.
In wafer inspection by such SEMs with a metrology function, or so-called CD-SEMs, a secondary electron image is obtained by image processing secondary electron signals obtained by scanning a wafer with an electron beam, and the shape of the pattern is determined based on changes in the brightness thereof, thereby deriving the dimensions of the pattern under inspection. SEMs comprise a sample stage device for holding a sample. Sample stage devices are configured to position a sample table, which is capable of two-dimensional movement and on which a wafer is mounted, in accordance with the site on the wafer that is to be observed.
SEM inspection of various patterns formed on wafers is required to accommodate, for example, 35 nm node design rules, and obtaining a secondary electron image for an observation site on a wafer with little noise at a high observation magnification of ×300,000 or greater is an important issue. Further, improvements in the contrast of observed images by overlaying numerous secondary electron images on top of one another are also demanded. In order to meet such issues and demands, sample stage devices needed to suppress device vibration and drift (a phenomenon where the resting position of the sample table shifts over time) on a nm scale.
A wafer size currently often inspected is 300 mm. In line therewith, sample tables of sample stage devices for mounting wafers have also become quite large compared to before. At the same time, in order to improve throughput, sample stage devices must also move and position these large sample tables at high speed. With high-power drive mechanisms for moving such large sample tables, increases in temperature occur due to heat generated by motors and drive shafts.
Accordingly, in sample stage devices, as a means for preventing sample stage device drift caused by thermal expansion/contraction due to a rise in temperature resulting from such heat generation, there is a technique in which a 20 to 100 μm gap is provided between the sample table and its drive shaft, and the drive shaft is severed from the sample table when at rest (e.g., Patent Literature 1).
Further, there is a technique in which, during fine movement of the sample table, which is a weakness thereof due to the dead zone created by such a gap, the deviation of a measurement value, which is measured by a position detector that measures the current position of the sample table, from a pre-set target value is monitored, and, when the deviation from the target position falls to or below a certain value, the fine movement drive of the sample table is stopped (e.g., Patent Literature 2).
Through such techniques, sample stage devices provided on conventional electron microscope devices are able to perform positioning control of a sample table by an open loop using a pulse motor.